This invention pertains generally to method and apparatus for preventing surface contamination by deposition of particulate matter and more particularly to preventing deposition of particulate matter onto lithographic components such as reticles (masks) and wafers during lithographic use, fabrication, inspection, repair, handling and storage.
The ability to produce high quality microelectronic devices and reduce yield losses is strongly dependent upon maintaining the surfaces substantially defect-free. This is particularly true as design rules drive integrated circuits to finer feature size. Generally, surface defects can be related to particulate matter being deposited onto surfaces of reticles (masks) and wafer substrates during the various operations required to produce integrated circuits. The need to maintain these surfaces substantially free of particulate matter has long been recognized in the microelectronics industry and various schemes to do so have been proposed, such as those set forth in U.S. Pat. Nos. 5,373,806 and 5,472,550. The former discloses the use of thermal energy, such as the use of radiant energy, RF, or resistance heating, to substantially eliminate electrostatic attraction as a mechanism for particle transport and deposition during gas phase processing while the latter describes the use of the photophoretic effect to capture particles by projecting a laser beam inside the processing chamber along a trajectory that does not contact the substrate surface.
The concern about printable defects caused by particle deposition onto surfaces is of particular importance for the next generation of lithographies, including proximity x-ray lithography, direct-write and projection electron-beam lithography (SCALPEL), direct-write and projection ion-beam lithography, and extreme ultraviolet (radiation having a wavelength in the region of 3.5-15 nm) lithography (EUVL) which must provide for exclusion of particles with diameters greater than 0.01 .mu.m. The situation is exacerbated by the fact that for a beam of high energy radiation (photons, electrons, ions, or atoms), such as used for the aforementioned advanced lithographies, a pellicle which is customarily employed to protect lithographic reticles (masks) from particle deposition cannot be used. The protective benefit provided by a protective membrane such as a pellicle is negated by its deleterious effect on the beam of high energy incident radiation. By way of example, a half micron thick Si film will reduce the light intensity at 13 nm by 60%, which is an intolerable reduction for most lithographic applications. Coupled with this is the difficulty of forming a durable pellicle consisting of a 1/2 .mu.m Si film. In the case of electron lithography, the pellicle will absorb some of the electron current and, by inelastic scattering, introduce undesirable chromatic aberration into the electron beam and intolerable deviations in beam angle. While it is possible to produce organic polymeric materials in the proper thickness to form pellicles, they suffer from the disadvantage that they will decompose under the influence of high energy radiation, releasing volatile degradation products which, in turn, will coat optical surfaces and reduce their efficiency. Moreover, many of the advanced lithographic concepts must operate in a vacuum to reduce degradation of high energy radiation used for finer design rules consequently, the pellicle surface will be subjected to large changes in pressure (from 760 Torr to 5.times.10.sup.-4 Torr) over a surface area that may be as large as 100 cm.sup.2 and thus, forces larger than a thin organic membrane pellicle can withstand will be generated.
Because of the importance of protecting lithographic surfaces, such as reticles, from deposition of particulate matter for next generation lithographies alternative protection schemes such as clean encapsulation of the exposure chamber, protective gas blankets, and in-situ cleaning of mask surfaces are being investigated. However, each of these alternative schemes has disadvantages and none have been developed to the point of application.
What is needed is a means to protect lithographic surfaces, such as those of the reticle and wafer, from particle deposition without comprising lithographic performance or contaminating lithographic optical elements. Moreover, in order to be useful in advanced lithographic applications it is necessary that the protecting means operate effectively in a sub-atmospheric pressure environment.